Fewer oak processionary caterpillars thanks to pheromone trick: Results of five-year study published |
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Following an intensive study that began in 2020, the results of the oak processionary moth mating disruption trial have now been published. Hellingman Advies, in collaboration with pheromone specialist Teun Dekker and field expert Gerard Brand from Brand Boomverzorging, spent five years on this research. Trade magazine Boomzorg presents the results and the process that preceded them.
Mating disruption is a method designed to confuse male oak processionary moths with the sex pheromones of females, preventing them from locating a mate. The results—and the accompanying final evaluation of the trial—provide a clear overview of the work carried out from 2020 to 2025.
The first field trial began in the 2020-2021 season. Silvia Hellingman of Hellingman Advies explains: "We had been working for years to set up this study," she says. "Because we started in the middle of the COVID pandemic, it took quite a while before we got all the supplies and could finally get going."
Early success
One thing that stands out in the final evaluation is that already after the initial trial in 2020 there was a significant decrease in the number of oak processionary moth nests, with the note that mating disruption proved more successful in sheltered locations.
"For a reasonably sheltered residential area, this is an ideal solution," says Hellingman. "But for trees exposed to open wind, the effect is far less. Those are facts that must be taken into account."
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"We were closely monitored by law enforcement."
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 | | Silvia Hellingman |
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Lessons learned
Pheromone-based mating disruption is a labor-intensive method. During the first trial, the researchers applied the pheromones manually. Equipped with caulking guns and a cherry picker, they treated no fewer than 1,509 trees.
For the second trial, they took a slightly different approach.
"After the first trial, we decided to use a paintball gun to apply the pheromones," Hellingman explains. "That was quite a process, because we needed a firearms permit from the police. Once we had that, we were also closely monitored by law enforcement."
 | | Photography: Project leader Silvia Hellingman |
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The results
Over the four years of field trials, the method was proven effective, according to the final evaluation. The active substance, the confusion pheromone, is only detected by male oak processionary moths.
The evaluation also confirms that only the male moths respond to the female's sex pheromone, making the method species-specific.
Hellingman adds that this method is cheaper than conventional control methods. "We're currently seeing very low activity from the oak processionary moth, but that can be misleading," she explains. "There are still many pupae underground, but no one knows for how long. Just look at the pine processionary moth—it can stay underground for up to seven years."
Field trials in England and Germany conducted between 2023 and 2024 showed results consistent with those in the Netherlands.
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"It doesn't get more sustainable than this."
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Comparing with declining pest pressure
The oak processionary moth has been declining for several years. According to a news report from the Oak Processionary Moth Knowledge Centre, a low pest pressure is expected again for the coming year, with only local hotspots.
Do the mating disruption results simply confirm what has already become apparent?
Hellingman states that there are always fluctuations in time and location, with some periods showing high pest pressure and others not.
The results, she emphasizes, were based on trials conducted exclusively in high-pressure areas.
"It's not like we only did research in low-pressure zones," she says. "Hotspots remain, so we developed this tool as a supplement to other measures."
In the final year of the study, pest pressure at several sites was so low that statistically valid conclusions could not be drawn.
What now?
Even after the trials, the story isn't over. Several matters still need to be resolved. "We need to fine-tune the formulation, and also address storage," Hellingman says.
And perhaps the most important issue is that the confusion pheromone has not yet been authorized in the Netherlands, meaning the method is not currently allowed.
The approval process is long and costly. "We can't take that on ourselves," Hellingman notes. "That lies with our government and Brussels.
If the method gets approved, it will be a perfect fit for biodiversity projects. The pheromone has no harmful effects on other species. It doesn't get more sustainable than this."
This article was originally published on April 29, 2025 on the Boomzorg website.
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